t7400: avoid path mangling issues

t7400: avoid path mangling issues

A recently introduced test uses an absolute path. But when run on Windows
using the MSYS bash, such a path is mangled into a Windows style path when
it is passed to 'git config'. The subsequent 'test' then compares the
mangled path to the unmangled version and reports a failure.

A path beginning with two slashes denotes a network directory
(//server/share path) and is not mangled. Use that trick to side-step the
issue. Just in case that 'git submodule init' regresses in such a way that
it accesses the URL, use a path name that is unlikely to exist on POSIX
systems, and that cannot be a server name on Windows.

Signed-off-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>